The present invention relates to an image transferring device for a copier, printer or similar electrophotographic image forming equipment and, more particularly, to a positional relation between a transfer bias section and a discharge section with respect to a sheet and control over the transfer bias in an image transferring device of the type transferring an image from an image carrier to a transfer belt while transporting the sheet and causing it to electrostatically adhere to the belt.
It is a common practice with image forming equipment to use an image transferring device of the type electrostatically transferring a toner image formed on an image carrier, or photoconductive element, to a sheet carried on a transfer belt to which an electric field opposite in polarity to the toner image is applied. This type of device usually includes an arrangement for applying a transfer bias to the transfer belt. For example, an electrode member is connected to a high-tension power source and held in contact with the rear of the belt at an image transfer position. Such an arrangement is advantageous over one which relies on a corona charger since it does not produce harmful ozone and can operate with a low voltage.
In addition to transferring a toner image from the photoconductive element to the sheet, the device with the above-stated bias arrangement deposits a polarized charge on the sheet by the transfer bias so as to cause the sheet to electrostatically adhere to the belt. Therefore, as the belt is moved, the sheet can be transported by the belt and separated from the belt due to the electrostatic adhesion.
However, when the sheet is caused to electrostatically adhere to the belt, it has to be separated from the belt after image transfer. For the separation of the sheet, use may be made of a transfer belt having a resistance of 10.sup.10..OMEGA..cm to 10.sup.13..OMEGA..cm, and a discharge member located downstream of an image transfer position with respect to an intended direction of movement of the belt for dissipating the charge of the belt, as disclosed in Japanese Patent Laid-Open Publication No. 83762/1988 by way of example. The discharge member reduces or cancels the charge of the sheet to promote easy separation of the sheet. Regarding the discharge of the belt, Japanese Patent Laid-Open Publication No. 96838/1978, for example, teaches an arrangement which uses a transfer belt having a resistance of 10.sup.8 .OMEGA..cm to 10.sup.13 .OMEGA..cm and, in the event of continuously transferring images from a plurality of photoconductive elements to a sheet carried on the belt, dissipates a charge of the belt deposited by a discharge ascribable to the separation of the sheet from one photoconductive element before the belt faces the next element.
On the other hand, when the transfer bias is maintained constant, a current to flow to the photoconductive element changes relative to the bias set at the transfer belt side due to changes in temperature, humidity and other environmental conditions. For example, in a high temperature and high humidity environment, an excessive current is apt to flow to the photoconductive element since the belt and sheet absorb moisture to lower their resistances. This increases the charge deposited on the photoconductive element and often causes the sheet to wrap around the element. In the opposite environment, the transfer of a toner image becomes defective. In the light of this, use may be made of control circuitry having a controller for controlling the output current of a high-tension power source and to which a roller which supports the belt is connected, as taught in, for example, Japanese Patent Laid-Open Publication No. 231274/1991. The control circuitry detects the output current of the power source by the support roller via the belt and controls the output current in matching relation to a feedback current flowing through the support roller. With such control circuitry, it is possible to maintain the current to flow to the drum constant and thereby prevent the sheet from wrapping around the drum while eliminating defective image transfer.
However, simply selecting an electric characteristic with regard to the belt is not satisfactory when the transfer bias or the discharging operation is to be set as stated above. Particularly, it is necessary to eliminate the wrapping of the sheet, defective image transfer and incomplete sheet separation by adequately positioning the constituents of the image transfer device relative to each other and selecting adequate materials at the actual design stage. Moreover, for the control of the surface potential of the sheet via the belt, not only changes in environment but also other factors, e.g., changes in surface potential ascribable to changes in resistance which are in turn ascribable to irregularities in the quality of belts particular to the production line and the size of an image have to be taken into account. Should such changes be neglected, the amount of charge for setting up an electric field required for image transfer would change. This would not only degrade the quality of an image but also aggravate the defective sheet separation.